kernel/linux.git/include/uapi/linux/fcntl.h, branch v6.12.80

fhandle: expose u64 mount id to name_to_handle_at(2)

2024-09-05T09:39:17+00:00

Now that we provide a unique 64-bit mount ID interface in statx(2), we can now provide a race-free way for name_to_handle_at(2) to provide a file handle and corresponding mount without needing to worry about racing with /proc/mountinfo parsing or having to open a file just to do statx(2). While this is not necessary if you are using AT_EMPTY_PATH and don't care about an extra statx(2) call, users that pass full paths into name_to_handle_at(2) need to know which mount the file handle comes from (to make sure they don't try to open_by_handle_at a file handle from a different filesystem) and switching to AT_EMPTY_PATH would require allocating a file for every name_to_handle_at(2) call, turning err = name_to_handle_at(-EBADF, "/foo/bar/baz", &handle, &mntid, AT_HANDLE_MNT_ID_UNIQUE); into int fd = openat(-EBADF, "/foo/bar/baz", O_PATH | O_CLOEXEC); err1 = name_to_handle_at(fd, "", &handle, &unused_mntid, AT_EMPTY_PATH); err2 = statx(fd, "", AT_EMPTY_PATH, STATX_MNT_ID_UNIQUE, &statxbuf); mntid = statxbuf.stx_mnt_id; close(fd); Reviewed-by: Jeff Layton Signed-off-by: Aleksa Sarai Link: https://lore.kernel.org/r/20240828-exportfs-u64-mount-id-v3-2-10c2c4c16708@cyphar.com Reviewed-by: Jan Kara Reviewed-by: Josef Bacik Signed-off-by: Christian Brauner

uapi: explain how per-syscall AT_* flags should be allocated

2024-09-05T09:37:45+00:00

Unfortunately, the way we have gone about adding new AT_* flags has been a little messy. In the beginning, all of the AT_* flags had generic meanings and so it made sense to share the flag bits indiscriminately. However, we inevitably ran into syscalls that needed their own syscall-specific flags. Due to the lack of a planned out policy, we ended up with the following situations: * Existing syscalls adding new features tended to use new AT_* bits, with some effort taken to try to re-use bits for flags that were so obviously syscall specific that they only make sense for a single syscall (such as the AT_EACCESS/AT_REMOVEDIR/AT_HANDLE_FID triplet). Given the constraints of bitflags, this works well in practice, but ideally (to avoid future confusion) we would plan ahead and define a set of "per-syscall bits" ahead of time so that when allocating new bits we don't end up with a complete mish-mash of which bits are supposed to be per-syscall and which aren't. * New syscalls dealt with this in several ways: - Some syscalls (like renameat2(2), move_mount(2), fsopen(2), and fspick(2)) created their separate own flag spaces that have no overlap with the AT_* flags. Most of these ended up allocating their bits sequentually. In the case of move_mount(2) and fspick(2), several flags have identical meanings to AT_* flags but were allocated in their own flag space. This makes sense for syscalls that will never share AT_* flags, but for some syscalls this leads to duplication with AT_* flags in a way that could cause confusion (if renameat2(2) grew a RENAME_EMPTY_PATH it seems likely that users could mistake it for AT_EMPTY_PATH since it is an *at(2) syscall). - Some syscalls unfortunately ended up both creating their own flag space while also using bits from other flag spaces. The most obvious example is open_tree(2), where the standard usage ends up using flags from *THREE* separate flag spaces: open_tree(AT_FDCWD, "/foo", OPEN_TREE_CLONE|O_CLOEXEC|AT_RECURSIVE); (Note that O_CLOEXEC is also platform-specific, so several future OPEN_TREE_* bits are also made unusable in one fell swoop.) It's not entirely clear to me what the "right" choice is for new syscalls. Just saying that all future VFS syscalls should use AT_* flags doesn't seem practical. openat2(2) has RESOLVE_* flags (many of which don't make much sense to burn generic AT_* flags for) and move_mount(2) has separate AT_*-like flags for both the source and target so separate flags are needed anyway (though it seems possible that renameat2(2) could grow *_EMPTY_PATH flags at some point, and it's a bit of a shame they can't be reused). But at least for syscalls that _do_ choose to use AT_* flags, we should explicitly state the policy that 0x2ff is currently intended for per-syscall flags and that new flags should err on the side of overlapping with existing flag bits (so we can extend the scope of generic flags in the future if necessary). And add AT_* aliases for the RENAME_* flags to further cement that renameat2(2) is an *at(2) flag, just with its own per-syscall flags. Suggested-by: Amir Goldstein Reviewed-by: Jeff Layton Reviewed-by: Josef Bacik Signed-off-by: Aleksa Sarai Link: https://lore.kernel.org/r/20240828-exportfs-u64-mount-id-v3-1-10c2c4c16708@cyphar.com Reviewed-by: Jan Kara Signed-off-by: Christian Brauner

fcntl: add F_CREATED_QUERY

2024-08-19T11:44:34+00:00

Systemd has a helper called openat_report_new() that returns whether a file was created anew or it already existed before for cases where O_CREAT has to be used without O_EXCL (cf. [1]). That apparently isn't something that's specific to systemd but it's where I noticed it. The current logic is that it first attempts to open the file without O_CREAT | O_EXCL and if it gets ENOENT the helper tries again with both flags. If that succeeds all is well. If it now reports EEXIST it retries. That works fairly well but some corner cases make this more involved. If this operates on a dangling symlink the first openat() without O_CREAT | O_EXCL will return ENOENT but the second openat() with O_CREAT | O_EXCL will fail with EEXIST. The reason is that openat() without O_CREAT | O_EXCL follows the symlink while O_CREAT | O_EXCL doesn't for security reasons. So it's not something we can really change unless we add an explicit opt-in via O_FOLLOW which seems really ugly. The caller could try and use fanotify() to register to listen for creation events in the directory before calling openat(). The caller could then compare the returned tid to its own tid to ensure that even in threaded environments it actually created the file. That might work but is a lot of work for something that should be fairly simple and I'm uncertain about it's reliability. The caller could use a bpf lsm hook to hook into security_file_open() to figure out whether they created the file. That also seems a bit wild. So let's add F_CREATED_QUERY which allows the caller to check whether they actually did create the file. That has caveats of course but I don't think they are problematic: * In multi-threaded environments a thread can only be sure that it did create the file if it calls openat() with O_CREAT. In other words, it's obviously not enough to just go through it's fdtable and check these fds because another thread could've created the file. * If there's any codepaths where an openat() with O_CREAT would yield the same struct file as that of another thread it would obviously cause wrong results. I'm not aware of any such codepaths from openat() itself. Imho, that would be a bug. * Related to the previous point, calling the new fcntl() on files created and opened via special-purpose system calls or ioctl()s would cause wrong results only if the affected subsystem a) raises FMODE_CREATED and b) may return the same struct file for two different calls. I'm not seeing anything outside of regular VFS code that raises FMODE_CREATED. There is code for b) in e.g., the drm layer where the same struct file is resurfaced but again FMODE_CREATED isn't used and it would be very misleading if it did. Link: https://github.com/systemd/systemd/blob/11d5e2b5fbf9f6bfa5763fd45b56829ad4f0777f/src/basic/fs-util.c#L1078 [1] Link: https://lore.kernel.org/r/20240724-work-fcntl-v1-1-e8153a2f1991@kernel.org Reviewed-by: Jeff Layton Reviewed-by: Jan Kara Signed-off-by: Christian Brauner

fcntl: add F_DUPFD_QUERY fcntl()

2024-05-10T06:26:31+00:00

Often userspace needs to know whether two file descriptors refer to the same struct file. For example, systemd uses this to filter out duplicate file descriptors in it's file descriptor store (cf. [1]) and vulkan uses it to compare dma-buf fds (cf. [2]). The only api we provided for this was kcmp() but that's not generally available or might be disallowed because it is way more powerful (allows ordering of file pointers, operates on non-current task) etc. So give userspace a simple way of comparing two file descriptors for sameness adding a new fcntl() F_DUDFD_QUERY. Link: https://github.com/systemd/systemd/blob/a4f0e0da3573a10bc5404142be8799418760b1d1/src/basic/fd-util.c#L517 [1] Link: https://gitlab.freedesktop.org/wlroots/wlroots/-/blob/master/render/vulkan/texture.c#L490 [2] Signed-off-by: Linus Torvalds [brauner: commit message] Signed-off-by: Christian Brauner

fs: Pass AT_GETATTR_NOSEC flag to getattr interface function

2023-11-18T13:54:07+00:00

When vfs_getattr_nosec() calls a filesystem's getattr interface function then the 'nosec' should propagate into this function so that vfs_getattr_nosec() can again be called from the filesystem's gettattr rather than vfs_getattr(). The latter would add unnecessary security checks that the initial vfs_getattr_nosec() call wanted to avoid. Therefore, introduce the getattr flag GETATTR_NOSEC and allow to pass with the new getattr_flags parameter to the getattr interface function. In overlayfs and ecryptfs use this flag to determine which one of the two functions to call. In a recent code change introduced to IMA vfs_getattr_nosec() ended up calling vfs_getattr() in overlayfs, which in turn called security_inode_getattr() on an exiting process that did not have current->fs set anymore, which then caused a kernel NULL pointer dereference. With this change the call to security_inode_getattr() can be avoided, thus avoiding the NULL pointer dereference. Reported-by: Fixes: db1d1e8b9867 ("IMA: use vfs_getattr_nosec to get the i_version") Cc: Alexander Viro Cc: Cc: Miklos Szeredi Cc: Amir Goldstein Cc: Tyler Hicks Cc: Mimi Zohar Suggested-by: Christian Brauner Co-developed-by: Amir Goldstein Signed-off-by: Stefan Berger Link: https://lore.kernel.org/r/20231002125733.1251467-1-stefanb@linux.vnet.ibm.com Reviewed-by: Amir Goldstein Signed-off-by: Christian Brauner

exportfs: allow exporting non-decodeable file handles to userspace

2023-05-25T11:16:57+00:00

Some userspace programs use st_ino as a unique object identifier, even though inode numbers may be recycable. This issue has been addressed for NFS export long ago using the exportfs file handle API and the unique file handle identifiers are also exported to userspace via name_to_handle_at(2). fanotify also uses file handles to identify objects in events, but only for filesystems that support NFS export. Relax the requirement for NFS export support and allow more filesystems to export a unique object identifier via name_to_handle_at(2) with the flag AT_HANDLE_FID. A file handle requested with the AT_HANDLE_FID flag, may or may not be usable as an argument to open_by_handle_at(2). To allow filesystems to opt-in to supporting AT_HANDLE_FID, a struct export_operations is required, but even an empty struct is sufficient for encoding FIDs. Acked-by: Jeff Layton Acked-by: Chuck Lever Signed-off-by: Amir Goldstein Acked-by: Christian Brauner Signed-off-by: Jan Kara Message-Id: <20230502124817.3070545-4-amir73il@gmail.com>

mm/memfd: add F_SEAL_EXEC

2023-01-19T01:12:37+00:00

Patch series "mm/memfd: introduce MFD_NOEXEC_SEAL and MFD_EXEC", v8. Since Linux introduced the memfd feature, memfd have always had their execute bit set, and the memfd_create() syscall doesn't allow setting it differently. However, in a secure by default system, such as ChromeOS, (where all executables should come from the rootfs, which is protected by Verified boot), this executable nature of memfd opens a door for NoExec bypass and enables “confused deputy attack”. E.g, in VRP bug [1]: cros_vm process created a memfd to share the content with an external process, however the memfd is overwritten and used for executing arbitrary code and root escalation. [2] lists more VRP in this kind. On the other hand, executable memfd has its legit use, runc uses memfd’s seal and executable feature to copy the contents of the binary then execute them, for such system, we need a solution to differentiate runc's use of executable memfds and an attacker's [3]. To address those above, this set of patches add following: 1> Let memfd_create() set X bit at creation time. 2> Let memfd to be sealed for modifying X bit. 3> A new pid namespace sysctl: vm.memfd_noexec to control the behavior of X bit.For example, if a container has vm.memfd_noexec=2, then memfd_create() without MFD_NOEXEC_SEAL will be rejected. 4> A new security hook in memfd_create(). This make it possible to a new LSM, which rejects or allows executable memfd based on its security policy. This patch (of 5): The new F_SEAL_EXEC flag will prevent modification of the exec bits: written as traditional octal mask, 0111, or as named flags, S_IXUSR | S_IXGRP | S_IXOTH. Any chmod(2) or similar call that attempts to modify any of these bits after the seal is applied will fail with errno EPERM. This will preserve the execute bits as they are at the time of sealing, so the memfd will become either permanently executable or permanently un-executable. Link: https://lkml.kernel.org/r/20221215001205.51969-1-jeffxu@google.com Link: https://lkml.kernel.org/r/20221215001205.51969-2-jeffxu@google.com Signed-off-by: Daniel Verkamp Co-developed-by: Jeff Xu Signed-off-by: Jeff Xu Reviewed-by: Kees Cook Cc: Dmitry Torokhov Cc: Hugh Dickins Cc: Jann Horn Cc: Jorge Lucangeli Obes Cc: Shuah Khan Cc: David Herrmann Cc: kernel test robot Signed-off-by: Andrew Morton

vfs: add faccessat2 syscall

2020-05-14T14:44:25+00:00

POSIX defines faccessat() as having a fourth "flags" argument, while the linux syscall doesn't have it. Glibc tries to emulate AT_EACCESS and AT_SYMLINK_NOFOLLOW, but AT_EACCESS emulation is broken. Add a new faccessat(2) syscall with the added flags argument and implement both flags. The value of AT_EACCESS is defined in glibc headers to be the same as AT_REMOVEDIR. Use this value for the kernel interface as well, together with the explanatory comment. Also add AT_EMPTY_PATH support, which is not documented by POSIX, but can be useful and is trivial to implement. Signed-off-by: Miklos Szeredi

open: introduce openat2(2) syscall

2020-01-18T14:19:18+00:00

/* Background. */ For a very long time, extending openat(2) with new features has been incredibly frustrating. This stems from the fact that openat(2) is possibly the most famous counter-example to the mantra "don't silently accept garbage from userspace" -- it doesn't check whether unknown flags are present[1]. This means that (generally) the addition of new flags to openat(2) has been fraught with backwards-compatibility issues (O_TMPFILE has to be defined as __O_TMPFILE|O_DIRECTORY|[O_RDWR or O_WRONLY] to ensure old kernels gave errors, since it's insecure to silently ignore the flag[2]). All new security-related flags therefore have a tough road to being added to openat(2). Userspace also has a hard time figuring out whether a particular flag is supported on a particular kernel. While it is now possible with contemporary kernels (thanks to [3]), older kernels will expose unknown flag bits through fcntl(F_GETFL). Giving a clear -EINVAL during openat(2) time matches modern syscall designs and is far more fool-proof. In addition, the newly-added path resolution restriction LOOKUP flags (which we would like to expose to user-space) don't feel related to the pre-existing O_* flag set -- they affect all components of path lookup. We'd therefore like to add a new flag argument. Adding a new syscall allows us to finally fix the flag-ignoring problem, and we can make it extensible enough so that we will hopefully never need an openat3(2). /* Syscall Prototype. */ /* * open_how is an extensible structure (similar in interface to * clone3(2) or sched_setattr(2)). The size parameter must be set to * sizeof(struct open_how), to allow for future extensions. All future * extensions will be appended to open_how, with their zero value * acting as a no-op default. */ struct open_how { /* ... */ }; int openat2(int dfd, const char *pathname, struct open_how *how, size_t size); /* Description. */ The initial version of 'struct open_how' contains the following fields: flags Used to specify openat(2)-style flags. However, any unknown flag bits or otherwise incorrect flag combinations (like O_PATH|O_RDWR) will result in -EINVAL. In addition, this field is 64-bits wide to allow for more O_ flags than currently permitted with openat(2). mode The file mode for O_CREAT or O_TMPFILE. Must be set to zero if flags does not contain O_CREAT or O_TMPFILE. resolve Restrict path resolution (in contrast to O_* flags they affect all path components). The current set of flags are as follows (at the moment, all of the RESOLVE_ flags are implemented as just passing the corresponding LOOKUP_ flag). RESOLVE_NO_XDEV => LOOKUP_NO_XDEV RESOLVE_NO_SYMLINKS => LOOKUP_NO_SYMLINKS RESOLVE_NO_MAGICLINKS => LOOKUP_NO_MAGICLINKS RESOLVE_BENEATH => LOOKUP_BENEATH RESOLVE_IN_ROOT => LOOKUP_IN_ROOT open_how does not contain an embedded size field, because it is of little benefit (userspace can figure out the kernel open_how size at runtime fairly easily without it). It also only contains u64s (even though ->mode arguably should be a u16) to avoid having padding fields which are never used in the future. Note that as a result of the new how->flags handling, O_PATH|O_TMPFILE is no longer permitted for openat(2). As far as I can tell, this has always been a bug and appears to not be used by userspace (and I've not seen any problems on my machines by disallowing it). If it turns out this breaks something, we can special-case it and only permit it for openat(2) but not openat2(2). After input from Florian Weimer, the new open_how and flag definitions are inside a separate header from uapi/linux/fcntl.h, to avoid problems that glibc has with importing that header. /* Testing. */ In a follow-up patch there are over 200 selftests which ensure that this syscall has the correct semantics and will correctly handle several attack scenarios. In addition, I've written a userspace library[4] which provides convenient wrappers around openat2(RESOLVE_IN_ROOT) (this is necessary because no other syscalls support RESOLVE_IN_ROOT, and thus lots of care must be taken when using RESOLVE_IN_ROOT'd file descriptors with other syscalls). During the development of this patch, I've run numerous verification tests using libpathrs (showing that the API is reasonably usable by userspace). /* Future Work. */ Additional RESOLVE_ flags have been suggested during the review period. These can be easily implemented separately (such as blocking auto-mount during resolution). Furthermore, there are some other proposed changes to the openat(2) interface (the most obvious example is magic-link hardening[5]) which would be a good opportunity to add a way for userspace to restrict how O_PATH file descriptors can be re-opened. Another possible avenue of future work would be some kind of CHECK_FIELDS[6] flag which causes the kernel to indicate to userspace which openat2(2) flags and fields are supported by the current kernel (to avoid userspace having to go through several guesses to figure it out). [1]: https://lwn.net/Articles/588444/ [2]: https://lore.kernel.org/lkml/CA+55aFyyxJL1LyXZeBsf2ypriraj5ut1XkNDsunRBqgVjZU_6Q@mail.gmail.com [3]: commit 629e014bb834 ("fs: completely ignore unknown open flags") [4]: https://sourceware.org/bugzilla/show_bug.cgi?id=17523 [5]: https://lore.kernel.org/lkml/20190930183316.10190-2-cyphar@cyphar.com/ [6]: https://youtu.be/ggD-eb3yPVs Suggested-by: Christian Brauner Signed-off-by: Aleksa Sarai Signed-off-by: Al Viro

fcntl: fix typo in RWH_WRITE_LIFE_NOT_SET r/w hint name

2019-10-25T20:28:10+00:00

According to commit message in the original commit c75b1d9421f8 ("fs: add fcntl() interface for setting/getting write life time hints"), as well as userspace library[1] and man page update[2], R/W hint constants are intended to have RWH_* prefix. However, RWF_WRITE_LIFE_NOT_SET retained "RWF_*" prefix used in the early versions of the proposed patch set[3]. Rename it and provide the old name as a synonym for the new one for backward compatibility. [1] https://github.com/axboe/fio/commit/bd553af6c849 [2] https://github.com/mkerrisk/man-pages/commit/580082a186fd [3] https://www.mail-archive.com/linux-block@vger.kernel.org/msg09638.html Fixes: c75b1d9421f8 ("fs: add fcntl() interface for setting/getting write life time hints") Acked-by: Song Liu Signed-off-by: Eugene Syromiatnikov Signed-off-by: Jens Axboe